Not Applicable.
Not Applicable.
1. Field of the Invention
This invention is directed to circuits that interface a telephone line to a MODEM and to AC powered telephones and are fabricated using integrated circuit techniques and silicon based LEDs.
2. Prior Art
Isolation between the telephone line and AC house powered telephone circuits require an isolation barrier. Specifically, answering machines, cordless telephones, speaker phones, FAX machines, and MODEMs require the use of an electrical supply powered by a 60/50 Hz AC source which cannot be electrically connected to the telephone line through a hard wired connection. The ground of the AC powered circuit must be allowed to float with respect to the telephone line. To couple signals between the telephone line and an AC powered circuit a number of isolation devices can employed including transformers, capacitors, and opto-couplers. In addition, relays may also be employed to act as electrically isolated control switches such as an off hook switch in a MODEM. The interface circuit between the telephone line and an AC line powered circuit is referred to as a Data Access Arrangement or DAA.
Traditionally, signal isolation has been accomplished by using a 600 Ohm transformer. The biggest drawbacks in using transformers, however, are size and cost. Capacitors can be used as isolation devices but add undesirable capacitance between the isolated circuits unless the capacitance is very small, i.e. on the order of a few pico-farads. Also, capacitively isolated circuits are susceptible to transient disturb if the ground bus of one of the isolated circuits sees a rapid voltage change with respect to the ground bus of the second isolated circuit. This effect is due to the fact that a capacitor must pass charge from the output of one circuit to the input of the other circuit if the ground bus voltage difference between the two circuits changes. Finally, capacitively coupled telephone isolation circuit complexity increases over that using a transformer. However, with integrated circuit technology this disadvantage is greatly reduced.
The disturb problem associated with capacitively coupled signal isolation is not present in optically coupled signal isolation. Like the capacitively coupled isolator, the optically coupled isolator requires more circuitry than does a transformer coupled isolator.
In the traditional MODEM DAA which uses a transformer for signal isolation an opto coupler is employed for ring detection and a relay for the off hook condition. A Darlington connected bipolar transistor pair is typically used to sink the off hook current while providing a high signal impedance.
With the ability to provide complex circuits at a low cost, it is feasible to eliminate the bulky transformer and replace it with either an isolation capacitor or an opto coupler. This was illustrated in U.S. Pat. No. 5,438,210 which showed how SOI combined with silicon based LEDs could be use to fabricate a monolithic DAA. One difficulty with this approach is the lack of general acceptance of SOI technology and the high cost of the substrate material.
Other inventions show how opto couplers can be used in a discrete circuit format to perform the isolating function. The opto couplers in these inventions use commercially available opto-couplers which are based on GaAs technology for the LED and silicon for the detector technology. The problem with these approaches is the cost of making a circuit with several discrete integrated circuits.
This invention relates to an optically isolated DAA which eliminates the traditional signal isolation transformer. Applications include MODEMs, FAX machines, and AC line powered telephones such as but not limited to speaker phones, cordless telephones, and telephone answering machines.
It is the objective to show how even low efficiency LEDs can be used eliminate the signal transformer and how to integrate components that have traditionally been discrete. In particular, traditional discrete LEDs such as GaAs diodes can be replaced either with low efficiency silicon based diodes such as porous silicon, avalanche diodes, deposited silicon carbide diodes, etc. or with deposited polymer light sources or even deposited GaAs LEDs. LEDs that are placed directly onto the silicon integrated circuit will lower the cost over discrete LEDs. Furthermore, unlike that of U.S. Pat. No. 5,438,210, this invention seeks to unitize bulk silicon technology which is much more available and lower in cost than SOI technology. The key to using low efficiency LEDs is to digitally encode the signal and transmit it across the optically coupled barrier as a series of digital pulses rather than as an analog signal. This removes the stringent requirement of linearity and phase integrity needed for the analog signal, especially as it applies to MODEMs.
It is another object of this invention to reduce the number of LED-light detector pairs to two for off hook and ring signaling in addition to signal coupling. It is another objective to show how a MODEM DAA can be realized using only two silicon chips in a single package. And another objective is to show how a speaker phone, a voice messaging phone, and a cordless phone can be realized using two silicon chip circuits.
U.S. Pat. No. 5,369,687. This patent describes an opto coupler based DAA which uses a D to A and an A to D converter on the telephone line side to drive the transmit and receive opto couplers. Two additional couplers are shown for the ring signal and the off hook signal.
U.S. Pat. No. 5,465,298. This patent uses opto couplers to transmit and receive the telephone signal. Transmission through the opto couplers is direct with an additional light detector use as a feedback sensor to improve linearity of the opto coupler.
U.S. Pat. No. 5,555,293. This patent show a xe2x80x9ctranshybrid that incorporates optically-coupled isolation stagesxe2x80x9d. Optical coupling is by analog means with analog feedback using a second detector.
U.S. Pat. No. 5,528,686. This patent shows another example of a linear opto coupler based telephone line interface.
U.S. Pat. No. 5,751,803. This patent shows a method of AC coupling both the tip and ring signals into a transmit optocoupler
U.S. Pat. No. 5,500,895. This patent is an example of a capacitive type telephone line isolation DAA.
U.S. Pat. No. 5,086,454. This patent shows the building blocks of and analog type opto-coupler based DAA for a MODEM.
U.S. Pat. No. 5,224,154. This patent shows a micro-controller based DAA which uses opto-couplers to transmit the data signals and the ring signal.